netdev - Re: [RFC PATCH 06/21] mlx5: add header

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Date:   Thu, 18 Jun 2020 14:50:53 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, kernel-team@...com, axboe@...nel.dk,
        Govindarajulu Varadarajan <gvaradar@...co.com>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [RFC PATCH 06/21] mlx5: add header_split flag

On Thu, Jun 18, 2020 at 11:12:57AM -0700, Eric Dumazet wrote:
> 
> 
> On 6/18/20 9:09 AM, Jonathan Lemon wrote:
> > Adds a "rx_hd_split" private flag parameter to ethtool.
> > 
> > This enables header splitting, and sets up the fragment mappings.
> > The feature is currently only enabled for netgpu channels.
> 
> We are using a similar idea (pseudo header split) to implement 4096+(headers) MTU at Google,
> to enable TCP RX zerocopy on x86.
> 
> Patch for mlx4 has not been sent upstream yet.
> 
> For mlx4, we are using a single buffer of 128*(number_of_slots_per_RX_RING),
> and 86 bytes for the first frag, so that the payload exactly fits a 4096 bytes page.
> 
> (In our case, most of our data TCP packets only have 12 bytes of TCP options)
> 
> 
> I suggest that instead of a flag, you use a tunable, that can be set by ethtool,
> so that the exact number of bytes can be tuned, instead of hard coded in the driver.

Sounds reasonable - in the long run, it would be ideal to have the
hardware actually perform header splitting, but for now using a tunable
fixed offset will work.  In the same vein, there should be a similar
setting for the TCP option padding on the sender side.
-- 
Jonathan